In the prior art and especially in the field of radar, pulse envelope modulated rf signals have had their spectrum spread over a time interval for transmission. At a receiver the signals are time-compressed, to recover aproximately the shape of the signals prior to spectrum spreading.
In some prior jam-resistant communications systems that are called frequency-hopping signal systems, continuous wave pulses are selected for transmission with any of a plurality of starting frequencies, which can be pseudo-random. In other conventional signaling systems, such as those called time-hopping systems, waveforms are transmitted at times which are selected from among a plurality of time slots, with the selection of time slot being pseudo-random if desired. Still other prior apparatus employs a principle known as direct sequence PN spreading, in which a coded sequence of N chips is transmitted in a given time interval, N being the number of orthogonal waveforms that are possible. The sequence is randomly changed by a key generator.
One problem with some systems of the prior art has been that the ratio of peak power to average power has been very high, with the consequence that the rf transmitter must have a relatively high power capability. In other systems a great amount of equipment has been required, especially at the receiver.